This invention relates to a wheelchair, and in particular a wheelchair having a tiltable seat.
There exist several commercially available manually operated wheelchairs for individuals who benefit from sifting in a tilted position. When applied to a wheelchair, the term tilt generally refers to the seat surface having a capability of being put into a tilted position, thereby redistributing pressure experienced by the user""s skin surfaces. However, in all known existing embodiments, tilt is accomplished via a mechanism that has limitations.
The tilting operation in prior art manually operated wheelchairs is usually performed by a wheelchair attendant rather than by the rider himself or herself, and the tilt lock and control mechanism is usually mounted near the rear handgrips. Thus an attendant is required to tilt the seat.
Typically, the seat is made to tilt about a line in the plane of the seat, several inches forward of the intersection of the seat support surface and back support surface. This location is used because it roughly coincides with the vertical center of mass of an individual sitting in the wheelchair (when not tilted). In this case, the seat and back rotate about a point along the vertical center of mass, with rotation about a fulcrum located at the seat surface. The result of tilting the seat and back about this point is that relatively little effort is required to rotate the individual as rotation is about that is initially near the rider""s center of mass. However, this arrangement places certain limitations on the functionality of the wheelchair. The rider""s knees move upward as the seat is tilted. In most instances the knees move at least 5 inches upward as the seat is tilted to 45 degrees, thereby preventing the person from fitting their legs under a table or desk while in the tilted position.
An additional disadvantage of that approach is that as the rider is tilted back, the center of gravity of the rider shifts rearwardly. As the center of gravity approaches and traverses the rear wheel axis, the rider is at risk of falling backwards. The approach used in the prior art to avoid this is to place the rear wheels of the wheelchair relatively further to the rear of the frame than is the case for non-tilting wheelchairs. But as a result, the wheelchair is longer and is therefore less adapted to maneuver in restricted environments, such as a home or an office. This wheel location also prevents the user from accessing the rear wheels for use in manually propelling the wheelchair.
In another prior art approach, the seat and back still tilt about a point roughly located at the center of mass of the seated individual, but tilting is through an offset cam or through the use of glides positioned underneath the seat. While the effort required to tilt the seated individual may be somewhat less in this assembly, the user""s knees still move upward as the seat tilts, preventing access under a table or desk, as described previously.
It is an object of the present invention to provide a wheelchair, which is specifically designed so that the users knees move upwardly only slightly as the chair is tilted, therefore allowing for unobstructed use of a desk or table while in a tilted position.
It is a further object of this invention to provide a wheelchair specifically designed so that the centre of gravity moves rearward only minimally as the wheelchair is tilted. As the wheelchair tilts, the centre of gravity moves primarily in a downward direction, therefore facilitating a wheelchair where the rear wheels can be positioned more forwardly than in prior art tilting wheelchairs. This feature permits easy access to the rear wheels for users who wish to have hand access to the rear wheels.
It is a further object of this invention to provide a wheelchair specifically designed so that the user can activate the tilting mechanism without assistance, allowing the individual to change position independently, either increasing or decreasing the amount of tilt. The change in position should be achievable without assistance from another person or assistance from a remote power source such as an electric motor.
The inventors have determined that by altering the location of the tilt fulcrum, many of the objects of the invention can be achieved.
The invention comprises locating the fulcrum between 3xe2x80x3 and 7xe2x80x3 from the front edge of the seat, the preferred location being about 5xe2x80x3 rearward of the front edge of the seat surface. Tilting about a line located too close to the front edge of the seat will result in some wheelchair configurations being less than ideal. In particular, if the seat surface is rotated about a line that is less than 3xe2x80x3 from the front edge of the seat, seat depths longer than 17xe2x80x3 will result in the rear edge of the seat getting too close to the ground when the seat is tilted at 45 degrees. This problem is especially apparent when the height of the level seat surface is less than 15xe2x80x3 above the floor. Tilting from a point too far rearward from the front of the seat will result in the user""s knees moving too far upward as the chair tilts, making tables and desks inaccessible when the chair is in the tilt position.
Having regard to the vertical plane, the invention, locates the tilt fulcrum between 1xe2x80x3 and 4xe2x80x3 below the front edge of the seat, measured when the seat is in a level position. When the seat is tilted about a fulcrum located 4xe2x80x3 below the seat surface, the change in knee elevation is minimized as the seat is tilted.
When the seat tilts about the fulcrum location according to the present invention, the centre of mass of the rider moves downward as the seat is tilted. This in turn reduces the need to place the rear wheels too far back and improves overall stability of the wheelchair. Since the rear wheels can be positioned more forwardly, this wheelchair will be easier for the user to propel because he/she will be able to reach the rear wheels.
However, since the fulcrum according to the invention is forward of the rider""s"" centre of mass, additional uplift force is preferably provided to assist the rider in bringing the seat out of tilt. The invention uses a gas strut to accomplish this. As the weight of potential riders varies widely, the uplift force should also be adjustable so as to balance the load applied by the user sitting and tilting on the seat. Such adjustability is achieved by accommodating different positions of the gas strut so that it has more or less mechanical advantage, depending on the weight of the individual using the wheelchair. If the user is heavy, the gas strut is positioned so that it pushes upwardly at a point far from the fulcrum, thereby maximizing its"" mechanical advantage. If the user is lighter, the gas strut may be positioned so that it pushes upwardly at a point close to the fulcrum, minimizing the struts"" mechanical advantage. Adjustable mounting brackets are provided with a plurality of holes allowing securement of the gas strut in the most advantageous position.
It will be appreciated that the inventors have achieved an adjustable uplift force that allows the same gas strut to be used in tilt chairs carrying a wide range of riders of different weight. Since the position of the gas strut can be changed to increase or decrease the effective mechanical advantage, the strut can be positioned in such a way that it nearly balances the weight of the user thereby allowing most riders to independently tilt the seat.
Other features of the invention will be appreciated by reference to the detailed description of the preferred embodiment and to the claims.